The geology of 433 Eros

Abstract

Abstract-The global high-resolution imaging of asteroid 433 Eros by the Near-Earth Asteroid Rendezvous (NEAR) Shoemaker spacecraft has made it possible to develop the first comprehensive picture of the geology of a small S-type asteroid. Eros displays a variety of surface features, and evidence of a substantial regolith. Large scale facets, grooves, and ridges indicate the presence of at least one global planar structure. Directional and superposition relations of smaller structural features suggest that fracturing has occurred throughout the object. As with other small objects, impact craters dominate the overall shape as well as the small-scale topography of Eros. Depth/diameter ratios of craters on Eros average -0.13, but the freshest craters approach lunar values of -0.2. Ejecta block production from craters is highly variable; the majority oflarge blocks appear to have originated from one 7.6 km crater (Shoemaker). The interior morphology of craters does not reveal the influence of discrete mechanical boundaries at depth in the manner of craters formed on lunar mare regolith and on some parts of Phobos. This lack of mechanical boundaries, and the abundant evidence of regolith in nearly every high-resolution image, suggests a gradation in the porosity and fracturing with depth. The density of small craters is deficient at sizes below -200 m relative to predicted slopes of empirical saturation. This characteristic, which is also found on parts ofPhobos and lunar highland areas, probably results from the efficient obliteration of small craters on a body with significant topographic slopes and a thick regolith. Eros displays a variety ofregolith features, such as debris aprons, fine-grained "ponded" deposits, talus cones, and bright and dark streamers on steep slopes indicative ofefficient downslope movement ofregolith. These processes serve to mix materials in the upper loose fragmental portion ofthe asteroid (regolith). In the instance of "ponded" materials and crater wall deposits, there is evidence of processes that segregate finer materials into discrete deposits. The NEAR observations have shown us that surface processes on small asteroids can be very complex and result in a wide variety of morphologic features and landforms that today seem exotic. Future missions to comets and asteroids will surely reveal still as yet unseen processes as well as give context to those discovered by the NEAR Shoemaker spacecraft.

Impact-Induced Seismic Activity on Asteroid 433 Eros: A Surface Modification Process

“…We infer from this that much of the regolith layer possesses a depth-dependent porosity and cohesion gradient, perhaps due to compaction from seismic shaking. This would produce lower porosity and higher cohesion with increasing depth (4). Such a gradient was observed for the lunar regolith, causing the regolith to preferentially slide at shallow critical depths (14).…”

“…This regolith layer displays evidence of downslope movement in several forms (1)(2)(3)(4): debris aprons at the base of steep slopes, bright streaks of freshly exposed material on crater walls, the pooling of regolith in topographic lows, a large number of degraded craters, and a scarcity of craters less than È100 m in diameter. One plausible explanation for these phenomena is seismic reverberation of the asteroid after impact events, which is potentially capable of destabilizing slopes, causing regolith to migrate downslope, and degrading or erasing small craters (1)(2)(3)(4)(5). Impact-induced seismic shaking of an asteroid in the 1 to 100 km size range is an attractive mechanism for three reasons.…”

“…Instead, we took advantage of Eros_s highly fractured global structure (4,18). Lunar seismic studies show that the dispersion of seismic energy in a fractured, highly scattering medium is a diffusion process, such that the seismic energy density e in a fractured asteroid should obey the equation (8)…”

Granular cohesion and fast rotators in the NEA population

“…There are a few of these monoliths of size greater than or equal to 200 m, but the remaining members range from 100 m down to the smallest sizes observed in the asteroid population. Note that the largest blocks observed on the asteroid Eros are of the order 100-150 m [17], potentially indicating this as the limiting size of competent boulders. The existence of such a maximum limit for a broken body is consistent with theories of material strength of bodies [1].…”

Looking into the evolution of granular asteroids in the Solar System

“…Even though it was originally assumed that the smallest of asteroids were monolithic rocks with a bare surface, the pictures obtained by NASA Galileo, Near-Showmaker (or just NEAR) and JAXA Hayabusa space missions revealed a substantial layer of unconsolidated rocky material and dust (regolith) covering the surface of (951) Gaspra, (243) Ida, (433) Eros [1,2] and (25143) Itokawa. Additionally, observations made by the Hubble Space Telescope of the disruption events of active asteroids P2013/R3 [3] and P2013/P5 [4] imply that not only asteroid surfaces are covered by regolith, but that their internal structure is not monolithic.…”